Hillary Clinton's campaign lashed out at the FBI on Saturday, saying there was no indication that a cache of recently discovered emails under review by the agency was connected to the Democratic nominee.
Barnstorming the West, Republican rival Donald Trump pounced on the reignited email controversy.
Clinton campaign chairman John Podesta told reporters FBI Director James Comey's letter to the US Congress about the new emails was "long on innuendo" and "short on facts".
"There's no evidence of wrongdoing," he said. "No charge of wrongdoing. No indication this is even about Hillary."
But on that latter point, Mr Comey in fact said the new trove was "pertinent" to the Clinton email investigation, without explaining how.
A government official said on Saturday that the Justice Department had advised the FBI against telling Congress about the new developments in the Clinton investigation because of the potential fallout so close to the election.
Justice officials concluded the letter would be inconsistent with department policy that directs against investigative actions that could be seen as affecting an election or helping a particular candidate, the official said.
Landing with a thud, the email issue again threatened to undermine an advantage built by Mrs Clinton, the Democratic nominee, over Mr Trump and raised the possibility that the Republican might be able to seize momentum during the final days before the November 8 election.
Mr Trump told a crowd in Golden, Colorado, on Saturday that the FBI's review of Clinton email practices raises "everybody's deepest hope that justice, as last, can be properly delivered". His crowd cheered Mrs Clinton's email woes, which Mr Trump has taken to calling the biggest political scandal since Watergate.
Mr Trump also had battleground Arizona on his plate Saturday. Early voting has been under way for weeks and Mrs Clinton, who was campaigning Saturday in Florida, has led in preference polls, both nationally and in key swing states.
The FBI is looking into whether there was classified information on a device belonging to Anthony Weiner, the disgraced ex-congressman who is separated from long-time Clinton aide Huma Abedin.
Mr Comey, in his letter to Congress on Friday, said the FBI had recently come upon new emails while pursuing an unrelated case and was reviewing whether they were classified.
The announcement raised more questions than answers and generated criticism that Mr Comey was injecting a significant development too close to an election. Yet the FBI director also faced the prospect of intense scrutiny if voters learned that he had been sitting on a major development until after the election.
Mr Podesta and campaign manager Robby Mook spoke to reporters in a conference call, following Mrs Clinton's hastily arranged news conference on Friday night in a high school choir room in Des Moines, Iowa.
In it, she said "the American people deserve to get the full and complete facts immediately. The director himself has said he doesn't know whether the emails referenced in his letter are significant or not".
But long-term, the development all but ensured that, even should Mrs Clinton win the White House, she and several of her closest aides would celebrate a victory under a cloud of investigation.